The definition of atomic mass says that it is the mass of one atom of the element with respect to 1|12 of the mass of carbon-12 atom.

My question is that why there is need to compare the atomic mass with carbon-12 or any other element , why not just we measure the mass of 1 atom of an element and call it as the atomic mass.

Please do not mark this question as duplicate because I have searched the web but didn't find any appropriate answer.

Any help is appreciated.

  • $\begingroup$ Please read this reference with historical point of view. $\endgroup$ Apr 14, 2018 at 9:46
  • $\begingroup$ Not sure if I get this question right but if I call the mass of 1 atom hydrogen 1 atomic mass and I also call the mass of one atom helium 1 atomic mass and also 1 atom fluorine is one atomic mass then one atom mass isn't defined at all because every atom weights one atomic mass while in reality they all have a different weight. $\endgroup$
    – DSVA
    Apr 14, 2018 at 10:34
  • 1
    $\begingroup$ @DSVA I think he's just asking why is there's amu at all, and not just in SI unit. OP should put it more clear and not taunt me with supposed unduplicability of the question ;) $\endgroup$
    – Mithoron
    Apr 14, 2018 at 16:52
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1 Answer 1


The reason that we compare the mass of atoms to one standard atom is because of the scale of atomic mass.

The mass of one atom is super small. For example, the mass of one carbon atom is $1.99 \times 10^-23$g, which in full is 0.0000000000000000000000199g. This clearly is a very long number, and so is inconvenient in working with.

To address this inconvenience, a standard atom needs to be set to compare others. It was first hydrogen having an atomic weight of 1, though was discontinued since non-integer masses were given for certain elements (e.g. C - 11.9, O - 15.9). Oxygen was then used from its chemical significance, which worked well. However, in 1961 carbon-12 was decided to be the standard between an international agreement of scientists and physicists.


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